The research objective is to evaluate microwave radiometers as a means of sensing sub-cutaneous temperatures in humans in a non-invasive manner. Microwave radiometry is to be used in clinical tests with patients believed to have breast tumors, and other conditions associated with localized, subsurface temperature increase or decrease. Electromagnetic radiation of several centimeters wavelength (microwaves) can penetrate the body to a depth of several centimeters, depending upon tissue water content. Hence the body's microwave thermal radiation can also escape from that depth, and its detected intensity indicates the tissue temperature at that depth. The proposed technique is thus passive and sends no radiation into the body. Laboratory tests with cats and humans demonstrate microwave detection of local subsurface temperature elevation, and imply the feasibility of the proposed clinical tests. A supporting program of radiometer and antenna development is also to be carried out. This program will depend on clinical measurement experience to indicate desirable improvements in radiometer and antenna performance.